1. Field of the Invention:
This invention relates generally to means utilized for reducing subsynchronous resonance in synchronous machines and more particularly is related to means for reducing subsynchronous resonance in synchronous machines for AC power system stability.
2. Description of the Prior Art:
In the analysis of power systems stability, it is well known that a synchronous machine connected to an infinite bus will oscillate or "hunt" under certain circuit conditions. In turbine driven generators, periodic variations in the torque applied to the generators caused periodic variations in speed or rotor oscillations. This results in periodic variations in voltage and frequency which are transmitted to the electrical power generating system. These mechanical periodic variations in torque, .omega..sub.M, (mechanical resonance frequency), cause modulation of the generator voltage which results in small side band components of the electrical system nominal electrical frequency, .omega..sub.S. It has been found that when a synchronous machine supplies power to a long transmission line to which series capacitors are connected for voltage regulation, the resultant line reactance may have a resonant frequency that may match the mechanical resonance frequency, .omega..sub.M that greatly amplifies the rotor oscillations (mechanical resonance frequency .omega..sub.M) referred to as negative damping. In this case, the lower, or subsynchronous side band component may cause an extremely high current to flow in the electrical system. This high current may feed back magnetically through the air gap of the generator so as to excite further the oscillation of the rotating mechanical apparatus and may cause considerable physical damage such as shaft breakage in the generator.
Although various methods have been proposed for stabilizing an electrical generating system, one of particular interest is the U.S. patent application Ser. No. 048,934, filed June 15, 1979, now U.S. Pat. No. 4,302,715, entitled "Dynamic Parallel Inductive Stabilizer For Synchronous Machines Having Torsional Oscillations" and assigned to the assignee of the present application. In this application, a delta-connected, thyristor-controlled, three-phase reactor bank is employed. The current in the reactor bank is modulated according to the torsional oscillation of the rotating mechanical system using the usual technique of thyristor conduction angle control. The modulating signal used to control the thyristors is derived by measuring the velocity variation of the generator shaft, using some mechanically coupled device, such as a tooth-wheel pick-up.